ESD Protection of ISFET sensors

ABSTRACT

Methods, apparatus and chip fabrication techniques are described which provide electrostatic discharge (ESD) protection to ion-sensitive field effect transistor (ISFET) based devices used to selectively measure ions in a liquid. According to one aspect of the invention, an ESD protection circuit, made up of conventional protective elements, is integrated onto the same silicon chip on which the ISFET is formed, along with an interface that is in contact with the liquid being measured and which does not open up paths for D.C. leakage currents between the ISFET and the liquid. According to a preferred embodiment of the invention, a capacitor structure is used as the interface between the protection circuit and the liquid sample. Further aspects of the invention are directed to methods per se for providing ESD protection for ISFET sensors utilizing the interface means (e.g, capacitor structure) referred to hereinabove, and processes for fabricating the novel interface on a silicon wafer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to methods and apparatus for (a)measuring ions in a liquid; (b) protecting such apparatus from theeffects of electrostatic discharge; and (c) fabricating such apparatus,including electrostatic discharge protection circuitry, on a siliconwafer.

More particularly, the invention relates to methods, apparatus and chipfabrication techniques which provide electrostatic discharge (ESD)protection to ion-sensitive field effect transistor (ISFET) baseddevices used to selectively measure ions in a liquid.

According to one aspect of the invention, an ESD protection circuit,made up of conventional protective elements, is integrated onto the samesilicon chip on which the ISFET is formed, along with a novel interface.According to the invention, the novel interface is a structure, incontact with the liquid being measured, which does not open up paths forD.C. leakage currents between the ISFET and the liquid.

According to a preferred embodiment of the invention, a capacitorstructure is used as the interface between the protection circuit andthe liquid sample.

Further aspects of the invention are directed to methods per se forproviding ESD protection for ISFET sensors utilizing the interface means(e.g, capacitor structure) referred to hereinabove, and processes forfabricating the novel interface on a silicon wafer.

2. Description of the Related Art

Methods and apparatus for measuring ions in a liquid using an ISFET arewell known to those skilled in the art. For example, Johnson, in U.S.Pat. No. 4,020,830, and Connery et al., in U.S. Pat. No. 4,851,104, bothincorporated herein by reference to illustrate the state of the art,propose the use of such apparatus. Such devices typically include ameasuring circuit and an ISFET immersed in a liquid for selectivelymeasuring ion activity therein.

The aforementioned devices have numerous applications, includingapplications in the medical and biomedical fields where it is well knownto use different ISFETs for measuring different ion activities, such as,for example, pH, pK and pNa.

While semiconductor FET type structures are known to be ESD sensitive,it was, until recently, believed by many that the ISFET structure waslargely insensitive to ESD effects because (1) the ISFET does notcontain a metallized gate electrode, (which usually is directly involvedin electrostatic damage), and (2) experience over time indicated thatsuch devices could be handled by many people without any evidence ofelectrostatic damage. Testing of ISFET electrodes has, however,indicated that large device shifts can occur following an ESD event.

Those skilled in the art will recognize that although attempts have beenmade to address the problem of preventing ESD damage in an ISFET basedsensors; the known approaches for solving this problem contain inherentlimitations, particularly when fabricating the sensor on a siliconwafer.

Before describing these limitations, examples of state of the artapparatus for protecting ISFET based sensors from ESD events will be setforth for comparison with the type of ESD protection methods andapparatus contemplated by the invention.

It will be demonstrated herein that ESD damage can be prevented (inaccordance with the teachings of the invention) by using protectioncircuitry (within an ISFET based probe assembly) that allows build up ofcharge in a test sample during an ESD event, while simultaneouslytransferring the charge to the ISFET's source, drain and substrate.

This approach to providing ESD protection minimizes the field developedacross the transistor's insulator structure by rapidly equalizingcharges on either side of the insulator during an ESD event.

An ESD damage protection circuit for performing the above function coulduse, for example, fast bi-directional zener diodes connected between thetest liquid and the transistor's source, drain and substrate conductors,as will be described in detail hereinafter with reference to thedrawing.

The electrical contact to the test liquid can be achieved using acounter electrode, such as the counter electrode is described in theaforementioned U.S. Pat. No. 4,851,104, previously incorporated hereinby reference.

Another example of state of the art apparatus for selectively measuringions in a liquid and protecting against ESD events, is set forth byLigtenberg et al., in U.S. Pat. No. 4,589,970. The U.S. Pat. No.4,589,970 patent is incorporated herein by reference for its descriptionof an ESD protection circuit used in an ISFET based sensor.

The ESD protection circuit taught in the incorporated U.S. Pat. No.4,589,970 patent comprises at least one electrode, connected via a lowimpedance contact to the liquid being sampled, coupled to the ISFET by aprotective element having a low impedance for high voltages and a highresistance to low voltages.

The incorporated U.S. Pat. No. 4,589,970 patent states thatunidirectional zener diodes, capacitors, mechanical switches and highthreshold voltage MOSFETS can be used in place of or in parallel withthe bi-directional zeners to protect against ESD events.

The use of discrete components mounted in the probe package to form theprotection circuit and/or the integration of the circuit onto theISFET's silicon substrate is proposed by the incorporated U.S. Pat. No.4,589,970 patent.

As for the limitations referred to hereinbefore, one road block to theintegration of the protection circuit described in the incorporated U.S.Pat. No. 4,589,970 patent (and any similar circuitry) onto the ISFET'ssilicon substrate is the difficulty in creating a metal electrode thatprovides a reliable low-impedance contact to the liquid.

The incorporated U.S. Pat. No. 4,589,970 patent suggests the use ofaluminum or polysilicon films to form the contact; however, both filmsare subject to chemical attack in many of the liquids that would bemeasured by the ISFET.

An alternative would be to deposit a film comprised of a noble metalsuch as gold or platinum for the contact. Unfortunately gold andplatinum films usually require an intermediate layer using materialssuch as titanium or chromium to provide good adhesion to the substrate;thus, the chemical resistance of the electrode could be compromised bythe addition of another film especially if pinholes occur in the noblemetal film.

Another concern that exists when a low impedance metal contact is usedbetween the test liquid (sample) and the protection circuit is thatpaths are opened up for D.C. leakage currents via the protection circuitbetween the sample and the ISFET source, drain and substrate.

Accordingly, it would be desirable if methods and apparatus wereavailable which provide ESD protection to ISFET based devices used toselectively measure ions in a liquid, using (a) an ESD protectioncircuit made up of conventional protective elements integrated onto thesame silicon chip on which the ISFET is formed; and (b) an interfacemeans, in contact with the liquid being measured, which does not open uppaths for D.C. leakage currents between the ISFET and the liquid.

Furthermore, it would be desirable to provide fabrication techniqueswhich would allow the aforementioned protection circuit to be integratedonto the ISFET's silicon substrate in a manner that avoids thedifficulty in creating a metal electrode to serve as a reliablelow-impedance contact with the liquid.

Further yet, it would be desirable to provide apparatus which utilizecontact films that are resistant to chemical attack in many of theliquids that would be measured by the ISFET, without having to resort tothe use of noble metals and intermediate layers to provide good adhesionfor the film to the substrate.

SUMMARY OF THE INVENTION

Accordingly it is an object of the invention to provide methods andapparatus for measuring ions in a liquid and which inherently protectthe apparatus from the effects of electrostatic discharge.

It is a further object of the invention to provide techniques forfabricating the aforementioned apparatus (i.e., apparatus for measuringions in a liquid integrated with electrostatic discharge protectioncircuitry), on a silicon wafer.

More particularly, it is an object of the invention to provideelectrostatic discharge protection to ISFET based devices used toselectively measure ions in a liquid.

Furthermore, it is an object of the invention to provide methods andapparatus which offer ESD protection to ISFET based devices used toselectively measure ions in a liquid, using (a) an ESD protectioncircuit made up of conventional protective elements integrated onto thesame silicon chip on which the ISFET is formed; and (b) an interfacemeans, in contact with the liquid being measured, which does not open uppaths for D.C. leakage currents between the ISFET and the liquid.

Still further, it is an object of the invention to provide chipfabrication techniques which would allow the aforementioned protectioncircuit to be integrated onto the ISFET's silicon substrate in a mannerthat avoids the difficulty in creating a metal electrode to serve as areliable low-impedance contact with the liquid.

Further yet, it is an object of the invention to provide ISFET basedapparatus for measuring ions in a liquid that utilizes non-metalliccontact films which are resistant to chemical attack, and which do notrequire the use of intermediate layers to provide good adhesion for thefilm to the substrate.

According to one aspect of the invention, an ESD protection circuit,made up of conventional protective elements, is integrated onto the samesilicon chip on which the ISFET is formed, along with a novel interface.The novel interface is a structure, in contact with the liquid beingmeasured, which does not open up paths for D.C. leakage currents betweenthe ISFET and the liquid.

According to a preferred embodiment of the invention, a capacitorstructure is used as the interface between the protection circuit andthe liquid sample.

According to one specific aspect of the invention, apparatus forselectively measuring ions in a liquid, comprises: (a) a measuringcircuit including a chemically sensitive ion sensor in the form of anion sensitive field effect transistor (ISFET) formed on a siliconsubstrate; (b) an electrostatic discharge (ESD) protection circuitintegrated onto the substrate; and (c) interface means, integrated ontothe substrate, for providing an interface between the protection circuitand the liquid, characterized in that the interface means provides acontact with the liquid without opening up paths for D.C. leakagecurrents between the ISFET and the liquid.

According to a preferred embodiment of the invention, the interfacemeans is a capacitor structure including (a) an electrode in electricalcontact with the protection circuit; and (b) a capacitor dielectric incontact with the electrode and the liquid being measured.

Furthermore, according to a preferred embodiment of the invention, (a)the aforementioned electrode is a metal film exhibiting thecharacteristic that its oxide is an insulator (examples of suitablemetal film include aluminum, antimony, hafnium, niobium, tantalum,tungsten, yttrium and zirconium); and the aforementioned capacitordielectric is the oxide of the metal chosen for use as the electrode.The term "metal film" as used herinafter is defined to mean a metal filmexhibiting the characteristic that its oxide is an insulator.

Still further, according to a preferred embodiment of the invention, theprotection circuit includes means for transferring charge built up in aliquid test sample as a result of an ESD event. The means fortransferring charge (to the source, drain and substrate of the ISFETincluded in the sensor), includes: (a) a first bi-directional zenerdiode between the aforementioned interface means and the source of theISFET; (b) a second bi-directional zener diode between the interfacemeans and the drain of the ISFET; and (c) a uni-directional zener diodebetween the interface means and the substrate.

Further aspects of the invention are directed to methods per se forproviding ESD protection for ISFET sensors utilizing the interface means(e.g, capacitor structure) referred to hereinabove, and processes forfabricating the novel interface on a silicon wafer.

In particular, one further aspect of the invention is directed to amethod for providing electrostatic discharge (ESD) protection to ionsensitive field effect transistor (ISFET) based ion selectiveelectrodes, comprising the steps of: (a) forming an ISFET circuit on asilicon chip; (b) integrating a protection circuit onto the chip onwhich the ISFET is formed; and (c) integrating, onto the chip, aninterface between the protection circuit and the liquid, wherein theinterface provides a contact with the liquid without opening up pathsfor D.C. leakage currents between the ISFET and the liquid.

Fabrication processes contemplated by yet another aspect of theinvention include a process for fabricating a capacitor to serve as aninterface between a liquid being measured and a protection circuitincluded on an ion sensitive field effect transistor (ISFET) chip,wherein the ISFET chip is used to measure ions in the liquid and furtherwherein the chip includes a silicon substrate, a field oxide layer andat least one chemical barrier layer, and a diffused P+ region forconnecting the capacitor to the protection circuit, comprising the stepsof: (a) opening a via in the field oxide and the at least one chemicalbarrier layer; (b) sputter depositing a metal film onto the chip tocreate a lower electrode for the capacitor; (c) connecting the film tothe diffused P+ region through the via; and (d) forming the oxide ofsaid metal film to serve as the dielectric for the capacitor.

The invention features ISFET subassemblies, built in accordance with theteachings of the present invention, that when used in sensor probes havesurvived multiple exposure to ESD levels of 20,000 volts when tested inaccordance with the IEC 801-2 standard.

Furthermore, the invention features chip fabrication techniques whichwould allow the aforementioned protection circuit to be integrated ontothe ISFET's silicon substrate in a manner that avoids the difficulty increating a metal electrode to serve as a reliable low-impedance contactwith the liquid sample being tested. Still further, the use of anon-metallic contact film, as contemplated by the invention, resistschemical attack, and does not require the use of intermediate layers toprovide good adhesion for the film to the substrate.

These and other objects, embodiments and features of the presentinvention and the manner of obtaining them will become apparent to thoseskilled in the art, and the invention itself will be best understood byreference to the following Detailed Description read in conjunction withthe accompanying Drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 depicts an illustrative protection circuit, within an ISFET basedprobe assembly, that allows build up of charge in a liquid test sampleduring an ESD event, while simultaneously transferring the charge to theISFET's source, drain and substrate.

FIG. 2 depicts a protection circuit for an ISFET based sensor thatincludes interface means in the form of the capacitor structure of thetype contemplated by a preferred embodiment of the invention.

FIG. 3 depicts the structure of an exemplary interface capacitor, of thetype contemplated by one aspect of the invention, as fabricated on asilicon wafer.

FIG. 4 depicts an example of how the ISFET, protection circuit andinterface means combination contemplated by the invention may beimplemented on silicon substrate.

DETAILED DESCRIPTION

As indicated hereinabove, it is known that ESD damage can be preventedby using protection circuitry (within an ISFET based probe assembly)that allows build up of charge in a test sample during an ESD event,while simultaneously transferring the charge to the ISFET's source,drain and substrate. Such an approach minimizes the field developedacross the transistor's insulator structure by rapidly equalizingcharges on either side of the insulator during an ESD event.

An exemplary protection circuit (within an ISFET based probe assembly),that allows build up of charge in a test sample during an ESD event,while simultaneously transferring the charge to the ISFET's source,drain and substrate, is illustrated in FIG. 1.

As shown in FIG. 1, the exemplary protection circuit (which is similarto the circuit described in the incorporated U.S. Pat. No. 4,589,970patent), uses fast zener bi-directional diodes 101-103, connected totest liquid 199 and the source (151), drain (152) and substrate (153)conductors of ISFET 150. The electrical contact to test liquid 199 isachieved with the use of a counter electrode (185) which, as indicatedhereinbefore, may be realized in accordance with teachings of U.S. Pat.No. 4,851,104, previously incorporated herein by reference.

The operation of the exemplary protection circuit depicted in FIG. 1 maybe summarized as follows. In the event of an ESD occurrence, charge canbe expected to build up in test sample 199 until the zener breakdownvoltage is achieved.

Assuming a bi-directional zener voltage of a value substantially lessthan that needed to damage the device, once that voltage is exceeded, ineither direction, the charge will be conducted between counter electrode185 and the ISFET source, drain and substrate depicted in FIG. 1.Because differential insulator voltage is limited to the zener breakdown voltage, the insulator and ISFET sensor itself is protected.

Those skilled in the art will recognize that the interconnect distancebetween the liquid contact point through zener diodes 101-103, to thesource, drain, and substrate conductors, is important. Because theduration of an ESD event is of the order of magnitude of 30 nanoseconds,at an approximate speed of light of 1 foot per nanosecond, the pathwaybetween counter electrode 185 and silicon terminals must be limited to adistance of the order of magnitude of 2.4 inches, If this approximatedistance is exceeded, insufficient time is available to transfer chargebetween the counter and silicon electrodes to limit insulator fields to200 volts.

This distance requirement precludes the possibility of locating thezeners external to the electrode and strongly argues for functionintegration either on or in close proximity to the silicon chip.

As indicated hereinabove, U.S. Pat. No. 4,589,970, to Ligtenberg et al.(previously incorporated herein by reference), describes the use of thecircuit similar to that shown in FIG. 1. The incorporated U.S. Pat. No.4,589,970 patent also states that uni-direction zener diodes,capacitors, mechanical switches and high threshold voltage MOSFETS canbe used in place of or in parallel with the bi-directional zeners.

Furthermore, as indicated hereinbefore, the use of discrete componentsmounted in the probe package to form the protection circuit and/or theintegration of the circuit onto the ISFET's silicon substrate is alsoproposed by the incorporated U.S. Pat. No. 4,589,970 patent.

As pointed out in the Background of the Invention, a significant problemconcerning the integration of the protection circuit described in theincorporated U.S. Pat. No. 4,589,970 patent (and any similar circuitry)onto the ISFET's silicon substrate, is the difficulty in creating ametal electrode that provides a reliable low-impedance contact to theliquid.

The incorporated U.S. Pat. No. 4,589,970 patent suggests the use ofaluminum or polysilicon films to form the contact; however, as pointedout hereinbefore, both films are subject to chemical attack in many ofthe liquids that would be measured by the ISFET.

The alternative of depositing a film comprised of a noble metal for thecontact and the associated requirement for an intermediate layer usingmaterials such as titanium or chromium to provide good adhesion to thesubstrate, etc. (as described hereinbefore, along with other problemsassociated with using a noble metal contact), mitigate against the useof the noble metal alternative.

Instead, according to one aspect of the invention, an interface means(preferably a capacitor structure), is used to provide an interfacebetween the test liquid and the protection circuitry made up ofconventional circuit elements.

Reference should be made to FIG. 2 which, as indicated hereinabove,depicts a protection circuit that includes interface means contemplatedby the invention, combined with most of the same circuit elementsdescribed with reference to FIG. 1. These include bi-directional zenerdiodes 201 and 202 shown respectively between the interface means 203(shown as the preferred interface capacitor, which is referred tohereinafter as interface capacitor 203), and source 204, and drain 205;as well as a uni-directional zener diode 206, shown between interfacecapacitor 203 and the substrate (207).

During an ESD event, charge will build up in the liquid being tested(shown as 299 in FIG. 2), as described above with reference to FIG. 1.Interface capacitor 203 couples the ESD pulse to zeners 201, 202 and206. As the voltages across the zeners exceed their breakdown voltagesthe pulse is in turn coupled to the source, drain and substrate of ISFET250. The field across the ISFET 250 gate region is minimized.

The primary advantage of the use of a capacitor to provide an interfacebetween the liquid and the protection circuit is that no metal electrodeis in contact with the liquid.

The problem of choosing a suitable contact metal or combination ofmetals is eliminated. Another advantage of the use of a capacitor isthat D.C. leakage current paths that might exist between the liquid andthe ISFET source, drain or substrate via the protection circuit aresubstantially curtailed.

Reference should now be made to FIG. 3 which shows the structure of anexemplary interface capacitor (capacitor 300) which, in accordance withone aspect of the invention, is integrated on silicon substrate 340 withthe other ESD protection elements shown in FIG. 2.

According to a preferred embodiment of the invention, the lowerelectrode of capacitor 300 (i.e., electrode 301), consists of a sputterdeposited metal film. In FIG. 3, the metal film is shown connected tothe other circuit elements on the chip through a via opened in the fieldoxide (layer 302) and other deposited films (layer(s) 303) used aschemical barriers. Through chemical or anodic oxidation of the metalfilm, an oxide film layer (304) may be formed to serve as the capacitordielectric. According to the preferred embodiment of the invention, testliquid 399 is in contact with the capacitor dielectric.

Reference should now be made to FIG. 4 for an illustration of how thestructure of the ISFET and the protection circuit contemplated by theinvention can be implemented on silicon substrate 400.

FIG. 4 depicts two of the boron diffused P+ regions, 401 and 402,serving both as the source and drain respectively of the depicted ISFET.Regions 401 and 402 also serve as anodes for the protective zener diodesdepicted in FIG. 2. The other two boron diffused P+ regions illustratedin FIG. 4 (regions 403 and 404), serve as anodes for both thebi-directional and uni-directional zeners which are depicted in FIG. 2.

The cathode or background for the zeners is provided by the phosphorusdiffused N regions, 405 and 406. Selection of the dopant surfaceconcentration and the junction depth for the N region determines thezener breakdown voltage. The terminals for the source, drain andsubstrate of the ISFET are provided on the backside of the substrate.

Those skilled in the art will now readily appreciate (with reference toFIGS. 2-4), that according to one specific aspect of the invention,apparatus for selectively measuring ions in a liquid, includes: (a) ameasuring circuit including a chemically sensitive ion sensor in theform of an ion sensitive field effect transistor (ISFET) formed on asilicon substrate (where the ISFET may be formed on the substrate asshown in FIG. 4); (b) an electrostatic discharge (ESD) protectioncircuit (of the type described with reference to FIG. 2,) integratedonto the substrate (also as shown by way of example in FIG. 4); and (c)interface means, integrated onto the substrate, for providing aninterface between the protection circuit and the liquid, characterizedin that the interface means provides a contact with the liquid withoutopening up paths for D.C. leakage currents between the ISFET and theliquid.

It may also be seen with reference to FIGS. 3-4 that, according to apreferred embodiment, the invention contemplates a process forfabricating: (a) a capacitor to serve as an interface between a liquid(such as liquid 399 shown in FIG. 3) being measured; and (b) aprotection circuit included on an ion sensitive field effect transistor(ISFET) chip (such as the chip shown in FIG. 4), wherein the ISFET chipis used to measure ions in the liquid and further wherein the chipincludes a silicon substrate (shown, for example as substrate 340 inFIG. 3), a field oxide layer (302 in FIG.3) and at least one chemicalbarrier layer (303 in FIG. 3), and a diffused P+ region (305 in FIG. 3)for connecting the capacitor to the protection circuit, comprising thesteps of: (a) opening a via in the field oxide and the at least onechemical barrier layer; (b) sputter depositing a metal film onto thechip to create a lower electrode for the capacitor; (c) connecting thefilm to the diffused P+ region through the via; and (d) forming theoxide of the metal film to serve as the dielectric for the capacitor.All of the aforementioned steps have been previously described hereinwith reference to FIG. 3.

What has been described in detail hereinabove are methods, apparatus andchip fabrication techniques which meet all of the aforestatedobjectives. As previously indicated, those skilled in the art willrecognize that the foregoing description has been presented for the sakeof illustration and description only. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching.

The embodiments and examples set forth herein were presented in order tobest explain the principles of the instant invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe instant invention in various embodiments and with variousmodifications as are suited to the particular use contemplated.

In view of the above it is, therefore, to be understood that the claimsappended hereto are intended to cover all such modifications andvariations which fall within the true scope and spirit of the invention.

What is claimed is:
 1. Apparatus for selectively measuring ions in aliquid, comprising:(a) a measuring circuit including a chemicallysensitive ion sensor in the form of an ion sensitive field effecttransistor (ISFET) formed on a silicon substrate; (b) an electrostaticdischarge (ESD) protection circuit integrated onto said substrate; and(c) interface means, integrated onto said substrate, for providing aninterface between said protection circuit and said liquid, characterizedin that said interface means provides a contact with said liquid withoutopening up paths for D.C. leakage currents between the ISFET and saidliquid.
 2. Apparatus as set forth in claim 1 wherein said interfacemeans further comprises a capacitor structure.
 3. Apparatus as set forthin claim 2 wherein said capacitor structure further comprises:(a) anelectrode in electrical contact with said protection circuit; and (b) acapacitor dielectric in contact with said electrode and the liquid beingmeasured.
 4. Apparatus as set forth in claim 3 wherein said electrode isa metal film.
 5. Apparatus as set forth on claim 3 wherein saidcapacitor dielectric is an oxide of said metal film.
 6. Apparatus as setforth in claim 1 wherein said protection circuit further comprises:(a) afirst bi-directional zener diode between said interface means and thesource of said ISFET; (b) a second bi-directional zener diode betweensaid interface means and the drain of said ISFET; and (c) auni-directional zener diode between said interface means and saidsubstrate.
 7. Apparatus as set forth in claim 6 wherein said interfacemeans further comprises a capacitor structure.
 8. Apparatus forproviding electrostatic discharge (ESD) protection to an ion sensitivefield effect transistor (ISFET) based measuring device used to measureion activity in a liquid, comprising:(a) an ESD protection circuitfabricated on a silicon substrate, including:(a1) a first bi-directionalzener diode coupled to the source of said ISFET; (a2) a secondbi-directional zener diode coupled to the drain of said ISFET; and (a3)a uni-directional zener diode coupled to said substrate; and (b)interface means, integrated onto said substrate together with said ESDprotection circuit, for providing an interface between said protectioncircuit and said liquid without opening up paths for D.C. leakagecurrents between the ISFET and said liquid.
 9. Apparatus as set forth inclaim 8 wherein said interface means is connected in series with each ofsaid first bi-directional zener diode, said second bi-directional zenerdiode, and said uni-directional zener diode.
 10. Apparatus for providingelectrostatic discharge (ESD) protection to an ion sensitive fieldeffect transistor (ISFET) based sensor chip used to measure ion activityin a liquid test sample, comprising:(a) means for transferring charge,built up in said liquid test sample as a result of an ESD event, to thesource, drain and substrate of said ISFET; and (b) interface means,integrated onto said chip together with said means for transferringcharge, for providing an interface between said protection circuit andsaid liquid without opening up paths for D.C. leakage currents betweenthe ISFET and said liquid.
 11. Apparatus as set forth on claim 10wherein said interface means further comprises a capacitor structure.12. Apparatus for providing electrostatic discharge (ESD) protection toan ion sensitive field effect transistor (ISFET) based sensor chip usedto measure ion activity in a liquid test sample, comprising:(a) meansfor building up charge in said test sample during an ESD event; and (b)means for transferring charge built up in said liquid test sample, as aresult of said ESD event, to the source, drain and substrate of saidISFET.
 13. Apparatus as set forth in claim 12 wherein said means forbuilding up charge and said means for transferring charge are integratedonto said chip.